Ultrasound operates by creating an image from sound in three steps—producing a sound wave, receiving echoes, and interpreting those echoes to create an image. Invasive ultrasonic apparatus is known for imaging areas of the human body, for example, for guiding therapeutic instruments through a catheter to a field of view within a human body. For example, U.S. Pat. No. 5,704,361 to Seward et al. discloses a volumetric image ultrasound transducer underfluid catheter system.
FIGS. 2-9 and 11-12 and their attendant description, for example, suggest specific methods of intervention for imaging purposes in the vicinity of a human heart. To reach such an area of interest within a human body, an ultrasound imaging and hemodynamic catheter may be advanced via the superior vena cava to a tricuspid valve annulus. A distal end of a cylindrical body includes a guide wire access port, and a guide wire provides a means of assuring that the catheter reaches a target for imaging. A surgical tool may be fed through the catheter to the area imaged.
U.S. Pat. No. 6,572,551 to Smith et al. provides another example of an imaging catheter. Tools may be incorporated in an exemplary catheter, including suction, guide wire, or an ablation electrode.
U.S. Pat. No. 5,967,984 to Chu et al. describes an ultrasound imaging catheter with a cutting element which may be an electrode wire or a laser fiber. FIGS. 1 and 2 also describe a balloon 14 and a means to inflate the balloon. The balloon, for example, may be utilized to dilate a vessel having strictures imaged via the imaging catheter.
Other imaging catheters are known. For example, U.S. Pat. No. 6,162,179 to Moore teaches bending (using a pull wire) an acoustic window into a known and repeatable arc for improved three dimensional imaging. U.S. Pat. No. 6,306,097 to Park et al. discloses an ultrasound imaging catheter whereby a first lumen provides access for an ultrasound imaging catheter and a second lumen provides a working port for a tool. U.S. Pat. No. 5,505,088 to Chandraratna et al. teaches using a 200 MHz transducer in an ultrasonic microscope combined with a catheter as a delivery means for the microscope to provide imaging of myocardial tissue. According to Chandraratna et al., lower frequency ultrasound transducers can provide deeper penetration in the tissue but do not provide the image quality provided by higher frequencies.
In view of these references, it is suggested that ultrasound is a common imaging technique that can be used to visualize internal organs, and that various frequencies of ultrasound have various advantages and disadvantages, and that the applications that can be made of ultrasound can vary depending on the frequencies used. All the above-cited references are incorporated herein by reference in their entirety for understanding illustrated and discussed aspects and embodiments of devices and methods herein.
Ultrasound has many uses in medical applications. For example, ultrasound is routinely used during pregnancy to provide images of the fetus in the womb. Generally, a water-based gel is applied to the patient's skin, and a hand-held probe, called a transducer, is placed directly on and moved over the patient. The probe typically contains a piezoelectric element that vibrates when a current is applied. In ultrasound devices, a sound wave is typically produced by creating short, strong vibrational pulses using a piezoelectric transducer. The sound wave is reflected from tissues and structures and returns an echo, which vibrates the transducer elements and turns the vibration into electrical pulses. The electrical pulses are then sent to an ultrasound scanner where they are transformed into a digital image.
While general-purpose ultrasound machines may be used for most imaging purposes, certain procedures require specialized apparatus. For example, in a pelvic ultrasound, organs of the pelvic region can be imaged using either external or internal ultrasound. In contrast, echocardiography, which is used in cardiac procedures, can require specialized machines to take into account the dynamic nature of the heart.
Ultrasound has advantages over other imaging methods such as magnetic resonance imaging (MRI) and computed tomography (CT). For example, ultrasound is a relatively inexpensive compared to those techniques. Ultrasound also is capable of imaging muscle and soft tissue very well, can delineate interfaces between solid and fluid filled spaces, and shows the structure of organs. Ultrasound renders live images and can be used to view the operation of organs in real time. Ultrasound has no known long-term side effects and generally causes little to no discomfort to a patient. Further, ultrasound equipment is widely available, flexible, and portable. However, ultrasound does have some drawbacks. When used on obese patients, image quality is compromised as the overlying adipose tissue scatters the sound. The sound waves are required to travel greater depths, resulting in signal weakening on transmission and reflection back to the transducer. Even in non-obese patients, depth penetration is limited, thereby making it difficult to image structures located deep within the body. Further, ultrasound has trouble penetrating bone and, thus, for example, ultrasound imaging of the brain is limited. Ultrasound also does not perform well when there is gas present (as in the gastrointestinal tract and lungs). Still further, a highly skilled and experienced ultrasound operator is necessary to obtain and to interpret quality images, although software is known to assist in the interpretation process. These drawbacks do not, however, limit the usefulness of ultrasound as a medical diagnostic and treatment tool.